If you want a bit of a challenge and a climb, try the "Crown Range to Lake Wanaka/New Zealand route. Nearly all the climbing comes in the first 7 miles, and then it's mostly downhill to the lake. It's a little over 31 miles and the scenery is nice, as well. Highly recommended.

I'm not a great climber, so the first 7 miles took longer (for me) than the subsequent 24. :-D